ADU rules in Nampa

A detached ADU is a straight permitted use in every Nampa residential zone: the schedule of uses at 10-3-2 marks 'Accessory dwelling, detached' with a P in U (University), AG (Agricultural), RA (Suburban Residential), RS (Single-Family Residential), RD (Two-Family), RML, RMH and RP. The schedule's own key is explicit that a P means outright permission while a C would send the use to a conditional use permit under chapter 25. The unit has to sit on the same lot as a single-family detached dwelling and, per the 10-1-2.H definition, contain only one bedroom with no separate address and no separate utilities. Practical ceiling of one per lot: a single-family property may have only two accessory structures (plus a detached garage or carport if none is attached) and only one of those may be a detached guesthouse or in-law quarters.

Nampa recognizes only a DETACHED accessory dwelling. Title 10 contains no attached or internal ADU use anywhere, the 10-1-2.H definition is written for 'a separate structure', and the Planning and Zoning FAQ tells homeowners an ADU 'is detached from the primary residential dwelling unit' while an attached unit is either an addition (accessible to and from the main home, so not a separate dwelling) or a duplex. A genuinely separate attached or basement unit therefore reads as a two-family dwelling, which 10-3-2 allows only by conditional use permit in the RS single-family zone and does not allow at all in the AG or RA zones; duplexes are outright permitted in RD, RML, RMH and RP. Idaho Code 67-6541(2)(a) is written in the alternative, requiring covered cities to allow 'either one (1) internal accessory dwelling unit within the single-family dwelling or one (1) detached accessory dwelling unit per lot', so a detached-only city can satisfy the state floor without ever adding an internal option. The pending ZTA-00049-2026 amendment would open this up, replacing the detached-only definition with the state's ADU definition and adding a Title 10 chapter 29 whose 10-29-3.C reads 'An accessory dwelling unit may be internal to, attached to, or detached from a stand alone structure', but that text is a proposal awaiting a City Council vote, not current law.

The definition at 10-1-2.H caps 'the square footage of the accessory residential use within the detached structure' at 500 square feet of livable space and limits the unit to one bedroom, with no separate address and no separate utilities. Two further caps stack on top: 10-1-7.D.2 says the combined footprint of a property's detached accessory structures may not exceed the footprint of the principal structure, and each residential chapter caps detached accessory structure height at the height of the principal building or 22 feet, whichever is greater (10-8-4.A in RS, 10-7-3.A in RA, 10-10-4.A in RD). The 500 square foot cap sits far below the floor Idaho Code 67-6541(2)(d) requires covered cities to adopt by February 1, 2027, which bars limiting an ADU to less than 1,000 square feet or 75 percent of the primary dwelling. The conformance amendment that would fix this (ZTA-00049-2026) is moving but is not law: the Planning and Zoning Commission heard it on August 11, 2026, the City Council voted on August 17, 2026 to authorize the next hearing, and the Council public hearing is set for September 8, 2026 with the ordinance reading date still listed as to be determined.

Nothing in Title 10 conditions an ADU on the owner living on the property. The definition's only occupancy-adjacent limits are one bedroom, no separate address and no separate utilities, and a full-text sweep of the whole Nampa city code turned up no owner-occupancy clause for any residential use, at application or ongoing. Idaho Code 67-6541(2)(e) also requires covered cities to prohibit an owner-occupancy requirement for the primary dwelling or the ADU by February 1, 2027, so one cannot be added later. Separate from ADUs, 10-1-7.D.4 makes it unlawful to let anyone stay more than 30 days in an 'unauthorized dwelling unit', which 10-1-2 defines as vehicles, tents, boxes and accessory buildings such as garages and storage sheds; that targets unpermitted conversions, not an approved ADU.

Nampa's parking schedule at 10-22-6 is organized by 'Property's/Structure's Principal Use Type' and has no accessory dwelling row at all; its residential entry (2 spaces per dwelling unit on a single property, at least one covered or enclosed in RS) is keyed to the single-family or two-family principal use. The RS chapter's own rule is broader on its face: 10-8-9 says 'At least two (2) paved, off street parking spaces shall be provided per residential dwelling unit (e.g., house) in the RS zone', and the code calls an ADU 'a detached accessory residential dwelling unit', so a literal reading would add two spaces. The code never resolves the tension and the city's ADU building permit application says nothing about parking, so confirm with Planning and Zoning (208-468-4430) before you count on either answer. Idaho Code 67-6541(2)(b) forces the question by February 1, 2027: covered cities must prohibit requiring any ADU off-street or guest parking unless the principal dwelling has no off-street parking, or the street is not paved and designed or constructed for on-street parking, or the principal dwelling is within one-quarter mile of transit, an employment area, or commercial services. The pending ZTA-00049-2026 amendment would answer it with a new 10-29-4 requiring one off-street space for the ADU unless the principal property already provides its code-required parking, the adjacent street is designed and constructed to allow on-street parking, or the principal dwelling is within one-quarter mile of transit, an employment center or commercial services; that is proposed text, not current law.

No hearing is involved: a detached ADU carries a P in all eight residential zones and 10-3-2's key reserves the chapter 25 conditional use permit process for uses marked C. Because a detached accessory structure over 200 square feet must meet the zone's principal-building setbacks and take out a building permit (10-8-4.B in RS), the route is a staff-reviewed building permit, and Nampa publishes a dedicated 'Residential Accessory Dwelling Unit Building Permit Application' whose checklist routes plans through Building, Planning and Zoning, Engineering and erosion-control review. Since July 1, 2026 Idaho Code 67-6541(3) independently guarantees this: ADU projects meeting the jurisdiction's land use requirements 'shall be approved administratively and as a matter of right, without the need for discretionary approval'.

Nampa's code has no rental license, no short-term rental ordinance, no cap on renting an ADU and no primary-residence test; the only recurring limits are structural (one bedroom, no separate address, no separate utilities), which make the unit awkward to lease as a fully independent household but do not bar a tenancy. Since July 1, 2026 Idaho Code 67-6539 bars any city from prohibiting short-term rentals or imposing owner occupation, day limits, extra parking, numeric caps, spacing rules or a conditional use permit in residential zones, and subsection (5) states that no city 'shall require a license, fee, permit, certification, or registration to operate a short-term rental'; a city may require only smoke alarms, a fire extinguisher and carbon monoxide detector per floor, escape ladders above the ground floor, IBC-based occupancy limits and an informational handout. An ADU cannot be sold off separately, since it is an accessory structure on the same lot, and 10-3-2 note 1.c bars an ADU where multiple dwelling units already exist and requires the ADU to be removed or integrated into the new structure if two or more units are later built there.